The Night Sky

One of the things I like to stress in astronomy courses is the connection between what we study in the textbook, and what is up in the sky. Throughout this semester I will make reference to certain stars as examples of red supergiants, or extreme old “Population II Stars”

(like Arcturus, the bright red star in the western sky these evenings) or the Andromeda galaxy (in great position for seeing this autumn).

This lecture will also describe some of the most fundamental astronomical phenomena that exist.

First, let’s get our bearings. If you go out and stand in the Iowa countryside, it looks like you are in the middle of a plane (geometric term). If you want to describe the position of something in the sky, you can give its direction (north, east, south, west) and its angle above the horizon. The two angles you use to specify the position are the azimuth (north=0 degrees, east 90 degrees, south 180 degrees), and the altitude (right on the horizon =0 degrees, straight overhead is 90 degrees). This system of reference is called the horizon system.

► See Figure 1.14 for an illustration of this. (Transparency)

In the horizon system, we see objects in the sky do a number of things.

· The Sun rises in the east, reaches its highest altitude angle when it is in the south, and sets in the west.

· When the Sun sets it gets dark and we see the stars, the Moon, and the planets.

· The Moon rises in the east, reaches its highest altitude angle when it is due south, and sets in the west.

· The Moon rises at a different time every night, and appears against a different set of stars, or constellation.

· Here’s one that many people don’t realize. If you look at the constellations one hour after sunset (or one hour before sunrise), you see a different set every season. There is a slight night-to-night change, in that the stars seem to shift slowly to the west, so that after a few months you get a whole different set of stars in the night sky. This is illustrated in the star charts in the front and back of your textbook. This important phenomenon was well known to everyone prior to the invention of television and street lights. There are abundant references to it in Greek plays and Roman poetry.

Ø Illustration with starry night program

► Question for the audience: What is going on to cause this east-to-west motion of all the celestial objects, this rising in the east and setting in the west?

● Next let’s talk about the seasonal differences in the appearance of the sky. If we go out and look at the sky at 9 PM tonight, we will see Scorpius and Sagittarius in the south, the star Altair in the southeast, and Vega straight overhead. If we do this in February at 9PM, we see Orion and the bright star Sirius (brightest star in the sky) in the south, and the bright yellow star (same color as the Sun) Capella overhead. What is causing this change?

The answer is the orbital motion of the Earth around the Sun. We don’t see stars in the same direction as the Sun.

► Illustration with diagram, demonstration.

The Seasons, Hot and Cold

When one talks about seasons, most people don’t think about subtle differences in the appearance of the starry sky (although ancient people did; such was the origin of the term ``dog days’’). They think about differences in the temperature. August is hot, January is cold (here in Iowa). At a lesser level, they may think of differences in the length of day and night.

All of the seasonal changes were carefully noted in antiquity, and were well documented by the time the first civilizations came into existence about 4500 years ago.

As mentioned above, the most basic astronomical observation is that the Sun rises in the east, reaches its maximum altitude angle due south, and sets in the west. But there is more to it than that, and the differences were carefully noted a long, long time ago by people.

These days, the Sun rises a little north of east, is at an altitude angle of 57 degrees when it is due south (``crosses the meridian’’) and sets a little north of due west.

On September 23, the Sun will rise due east , be 48 degrees above the southern horizon at its highest, and set due west. This date is known as the autumnal equinox.

After that, the Sun will rise to the south of due east, reach a smaller angle above the southern horizon, and set to the south of west. Each day it gets further south. Look at Figure 2.12 in the book.

The furthest south the Sun gets is on about December 22, the winter solstice. On this day the Sun rises and sets furthest to the south, and reaches (here in Iowa City) a highest altitude angle of about 25 degrees.

After that, it begins marching north again, rising and setting due east and west, respectively on the vernal equinox about March 21. The furthest it rises to the north and sets to the north happens on the summer solstice, June 21. On June 21, the Sun is at an altitude angle of 71 degrees.

>>>>>>>>> Transparency showing all of this.

What is causing this behavior, with the same period as that of the Earth’s orbit around the Sun? Of course, when it was discovered, people had no idea that the Earth did go around the Sun.

Seasonal changes are due to:

The Earth rotates on its axis.
The rotation axis is not perpendicular to the plane of the Earth’s orbit.

>>>>>>>>>>>> Demonstration with Earth model.